Mouse embryonic stem (ES) cells are derived from the inner cell mass (ICM) of blastocysts and share similar gene expression patterns with the ICM. The defining features of ES cells are pluripotency and self-renewal, both of which have been the focus of intensive research for many years. Another hallmark of mouse ES cells is their ability to defy cellular senescence and to proliferate more than 250 doublings without crisis or transformation (Suda et al., J Cell Physiol 133:197-201, 1987). Although the fraction of euploid cells tends to decrease in long-term culture (Rebuzzini et al., Cytotechnology 58:17-23, 2008), the genome integrity of mouse ES cells is more strictly maintained than any other cultured cells. For example, ES cells maintain their ability to form chimeric animals with germline competency even after many passages (Longo et al., Transgenic Res 6:321-328, 1997; Nagy et al., Proc Natl Acad Sci USA 90:8424-8428, 1993; Sugawara et al., Comp Med 56:31-34, 2006). The mutation frequency in ES cells is also much lower (>100-fold) than those in mouse embryonic fibroblast cells and other somatic cells (Cervantes et al., Proc Natl Acad Sci USA 99:3586-3590, 2002). The unique feature of mouse ES cells can be further highlighted by a lower frequency of chromosomal abnormalities compared to embryonal carcinoma cells, which share similar characteristics to ES cells (Blelloch et al., Proc Natl Acad Sci USA 101:13985-13990, 2004), as well as some human ES cells (Brimble et al., Stem Cells Dev 13:585-597, 2004). However, the mechanism by which mouse ES cells maintain genomic stability is currently poorly understood.
Telomeres are repetitive DNA sequences accompanied by proteins that cap and protect the end of each chromosome from continuous degradation in each cell cycle, thereby securing and protecting chromosomal integrity. Telomere shortening may lead to cancer by contributing to genomic instability (Raynaud et al., Crit. Rev Oncol Hematol 66:99-117, 2008), and has been associated with aging and cellular senescence (Yang, Cytogenet Genome Res 122:211-218, 2008). Telomerase has been identified as the major enzyme known to be involved in telomere elongation maintenance. Although telomerase is active in ES cells (Thomson et al., Science 282:1145-1147, 1998), telomerase knockout ES cells (Terc−/−) show a marked reduction in telomere length only after 400 cell doublings, reaching a dramatic senescence event 10-30 doublings later, followed by establishment of a telomerase-independent population with no marked short telomeres (Niida et al., Nat Genet. 19:203-206, 1998; Niida et al., Mol Cell Biol 20:4115-4127, 2000). Hence, a telomerase-independent mechanism for telomere maintenance, named alternative lengthening of telomeres (ALT) (Bryan et al., EMBO J. 14:4240-4248, 1995), has been suggested for Terc−/− ES cells. Apparently, telomere recombination, or telomere sister chromatid exchange (T-SCE) can compensate for the lack of telomerase activity. Indeed, an increased frequency of T-SCE events has been demonstrated in long-term cultures of Terc−/− ES cells (Bailey et al., Nucleic Acids Res 32:3743-3751, 2004; Wang et al., Proc Natl Acad Sci USA 102:10256-10260, 2005).
Additionally, telomere recombination appears to be a normal mechanism in preimplantation embryos. Even though enzymatic activity of telomerase cannot be detected in preimplantation embryos from the unfertilized oocyte stage to the blastocyst stage (Wright et al., Dev Genet. 18:173-179, 1996), telomere length is rapidly increased during this period. In just one cell cycle, the average telomere length of 2-cell stage mouse embryos is doubled compared to that of unfertilized eggs (Liu et al., Nat Cell Biol 9:1436-1441, 2007). Although T-SCE events have been demonstrated in several studies, genes involved in this important process remain to be identified.